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Bellows Extension calculation

I've decided to use small tape measures(the itty bitty 3 footers) to check bello ws extension when I'm using both my lenses. I'll mark each inch and measure ext ension from the lensboard to the groundglass, unless somebody advises me that th is won't work.

I've read up on all the math for the calculations, but I'm not too proud to ask for you folks who are used to doing this, to take a 'looksee' at my math to see if these calculations are right.

Here is what I came up with....

I've decided to use small tape measures(the itty bitty 3 footers) to check bello ws extension when I'm using both my lenses. I'll mark each inch and measure ext ension from the lensboard to the groundglass, unless somebody advises me that th is won't work.

I've read up on all the math for the calculations, but I'm not too proud to ask for you folks who are used to doing this, to take a 'looksee' at my math to see if these calculations are right

Bellows Extension calculation

Well something is not quite right here - there is no way that 13" of bellows extension on a 12" lens should require an exposure increase of more than 1/2 stop. On the other hand at various points your calculations appear to be correct - notably where the increase is a whole number of stops. So, how are you calculating the intermediate points? As I say, something is not quite right.

The exact formula for the number of stops increase in exposure is:

[ log(bellows extension) - log(focal length) ]/ log(2),

(or there are plenty of other equivalent ways of arriving at the figure). Perhaps the most straight forward method is to simply divide the bellows extension by the focal length, and multiply this figure by the nominal f-number to get the effective f-number.

Bellows Extension calculation

And then there is the very easy way, which is to consider your lens focal lenght as an f stop and interpolate, so that if you have an 11 inch lens and you have a bellows extension of 16 inches you would open up a stop, and 22 inches 2 stops, etc. and in between you interpolate. It is not as accurate as all the math, but it works very well for me at least. try it, you might like it. Kevin

Bellows Extension calculation

Huw....yes, yes, something was/is wrong. I checked my math, filter factors were correct. I missed a step on calculating exposure increase which is supposed to be, ln(filter factor)/ln2=exposure increase. Thanks for the input. These are the corrected figures.

Bellows Extension calculation

Yes, that all looks right now. As others have suggested there are any number of ready-reckoning methods and devices you can buy, but you've picked the same method I used when starting out with a 35mm camera and extension tubes 27 years ago - just write down the pre-calculated figures in a table and keep it handy. Simple, cheap, and accurate! What more could anyone ask?

Bellows Extension calculation

Johnathan... a couple of tips. Put a lens on, focus to infinity. Measure the distance between the two lens standards. (I measure from outside of each standard, just be consistent) This is your infinity focus seperation, no compensation required. Record this value for each lens, it's critical. This also overcomes the issue of lens nodal point placement which you have overlooked, and at times can be quite significant.

Now, from this distance, lets say its 14", and the lens is 13" fl. Now you know each time you increase the standard seperation by 13" you add 2 stops. If you increase the seperation by half this amount, 6.5", then you compensate by 1/2 of 2 stops or 1 stop. If you add 2" to the infinity seperation distance, 2/13 = .15 x 2 stops = .3 stops. From this, you can make a simple cheat sheet which has 3 colums for both your lenses.

The beauty of this system is, it automatically accounts for the lens nodal point. Once you determine this, the math becomes simple, 2 stops for each fl distance you add to the infinity sepeation. You measure the seperation, look on your tiny cheat sheet and voila, perfect everytime. If you don't want to carry a cheat sheet, just tape the infinity seperation distance on each lens and do the math in your head after you take your mesurement, it's pretty simple.

The poster above who mentioned the discs, this is better suited for table top photography, but is not well suited for 1/3 stop compensation as the discs are hard to read. Also keep in mind when applying drastic shifts or tilts, you will be approaching the darker area of the image circle you need to compensate accordingly for this also.

The other benefit of using this system is it works perfect for telephoto lenses! You do the exact same procedure. As you know, there is no formula for the tele lenses, all the bellows compensation formulas are made for normal lenses. This "standard" seperation method compensates for everything. Hope this helps...

Bellows Extension calculation

To get really, really picky, the progression of bellows compensation from infinity focus (with no compensation) to any other extension is not linear, but logarithmic. For example, using the 13" lens just cited, you would need a 1 stop adjustment (factor of 2) at 13" x 1.414 or 18.38", instead of half-way between 13" and 26", or 19.5" Like I said, this is really picky, but if you are going to the trouble of constructing a chart with a factor calculated at every inch of extension as Jonathan did (which was right the second time around) you might want to use the formula.

On the other hand, you probably need to balance out how much precision you need in exposure calculation with how precisely you can control your development process.